Conventionally, a radio communication system has been known that includes base stations and radio network controllers. The base station includes one or more of cells, and each cell performs radio communication with radio terminals. The radio network controller manages multiple base stations and assigns a radio resource to a radio terminal. Note that such a technique (hereinafter, first technique) may be referred to as an R99 (Release 99) and the like.
In recent years, a technique has been proposed in which a base station (network side) performs the assignment of a radio resource for uplink data from a radio terminal to the base station and any other operation for the purpose of improving throughput and reducing delay time. Such a technique (hereinafter, second technique) may be referred to as HSUPA (High Speed Uplink Packet Access), EUL (Enhanced Uplink), and the like.
Each cell may function as a serving cell in some cases and as a non-serving cell in other cases. Transmission rate (for example, TBS (Transport Block Size) determined by SG (Scheduling Grant)) for uplink data is controlled by transmission rate control data transmitted from the serving cell and the non-serving cell. The transmission rate control data includes absolute transmission rate control data (AG; Absolute Grant) for directly specifying the transmission rate and relative transmission rate control data (RG; Relative Grant) for relatively specifying the transmission rate (for example, 3GPP TS25.321 Ver. 7.5.0).
Here, the uplink data is transmitted from a radio terminal to a base station through an E-DPDCH (Enhanced Dedicated Physical Data Channel). The absolute transmission rate control data (AG) is transmitted from the radio terminal to the base station through an E-AGCH (E-DCH Absolute Grant Channel). The relative transmission rate control data (RG) is transmitted from the radio terminal to the base station through an E-RGCH (E-DCH Relative Grant Channel).
The serving cell transmits the absolute transmission rate control data (AG) and the relative transmission rate control data (RG) to the radio terminal. On the other hand, the non-serving cell only transmits the relative transmission rate control data (RG) to the radio terminal without transmitting the absolute transmission rate control data (AG).
A base station according to the second technique includes a call admission control unit that controls whether or not to receive a new call and a scheduling unit that controls a radio resource (transmission rate) to be assigned for uplink data. The scheduling unit transmits absolute transmission rate control data (AG) or relative transmission rate control data (RG). As types of TTI (Transmission Time Interval) of uplink data, there are 2-msec TTI and 10-msec TTI corresponding to the length of 1 TTI. In the case of the 2-msec TTI, the scheduling unit can transmit the absolute transmission rate control data (AG) or the relative transmission rate control data (RG) at each TTI.
Here, a method can be considered in which; the scheduling unit measures the frequency of the occurrences of events where a target transmission rate is not achieved in a control cycle; and the call admission unit denies to receive a new call in the next control cycle when the frequency of the occurrences is high (for example, Japanese Patent Application Publication No. 2007-159054).
Unfortunately, in the second technique (EUL), even when the base station measures the transmission rate, the resulting transmission rate may fall below the target transmission rate due to a factor in an upper level device of the base station.
Moreover, in the second technique (EUL), even though the target transmission rate is achieved, shortage of radio resources to be shared with the first technique (R99) may occur to thereby adversely affect the quality of a call in the first technique (R99).
As described above, the admission of a new call has not been able to be appropriately controlled in accordance with the frequency of the occurrences of the events in which the target transmission rate is not achieved.